SUMMARY: We examine the efficacy of an online Social Intelligence Intervention (SII) at improving the health and well- being of custodial grandmothers (CGMs) and their adolescent custodial grandchildren (ACG) through mutual enhancement of their social competencies. This target population is of particular importance because both CGMs and ACG experience significant early life adversities that lead to hypervigilance of others, mistrust, social isolation, interpersonal conflict, and the inability to garner warmth and support from family and friends. Numerous studies have shown that these relational challenges within ?risky families? often lead to life-long interpersonal difficulties that increase the probability of behavioral and physical health problems. Furthermore, adolescence is a key period for the development of social competence, which is influenced by supportive caregiving and positive modeling from female parent figures. However, because the ability of CGMs to carry out this intergenerational transmission of social skills is challenged, examining joint social intelligence training for CGM-ACG dyads is valuable for reversing this negative sequela. Yet, to date, no other investigators have done so. To address this gap, we will conduct an online randomized clinical trial with 340 nationally-recruited CGM-ACG (ages 12-18) dyads assigned to either the SII or an attention control condition. Data will be obtained at pre- and post-test, and at 3-, 6-, and 9-month follow-ups via questionnaires completed by phone. Daily dairies will be collected online from 170 randomly selected dyads, and qualitative interviews will be conducted with 60 dyads to probe how the SII affected their daily social competence and social ties. We will obtain quantitative and qualitative measures of key social cognitive processes, quality of close interpersonal ties, psychological well-being, and physical health. Administrative medical, criminal, and education records for all 340 dyads will also be obtained for cost-benefit analyses that examine changes in burden on public systems. These mixed-methods allow rigorous examination of four specific aims: (1) To investigate if the SII enhances social competencies that, in turn, produce long-term changes in relationship quality, well-being, and physical health; this includes determining if increased social competence in one dyad member leads to partner effects in the other; (2) To examine if cumulative risk, gender, and age moderate SII efficacy; (3) To study qualitatively how CGM-ACG dyads view the SII as having changed their social competencies and yielded positive outcomes; and (4) To assess the financial benefits of the SII to participants and their communities. These aims address a highly significant public health problem that will inform future efforts to support a wide range of high risk families, like custodial grandfamilies, who typically have insufficient access to formal support services. Our SII is advantageous because it is inexpensive, delivered online, non-stigmatizing, and capable of reaching a target population that is geographically disperse and greatly in need of supportive programming.